Lecture 9 Yield and Yield Components
Components of Yield in Seed Crops
This section discusses critical aspects of yield and yield components in seed crops, which serve as a valuable tool for both research and practical applications for growers seeking to increase their yields or evaluate performance after experiencing poor yields.
Yield Components Overview
Yield can be broken down into several components explained by the acronym PENNS:
P: Number of plants per square meter.
E: Number of ears or stems on each plant.
N: Number of seeds on the seed head.
S: Seed weight.
Plant Density Challenges
Determining plant density often becomes complicated. For instance, in a ryegrass seed crop, it is easy to know the plant density shortly after planting, but as the plants grow, distinguishing one plant from another can become troublesome, especially by springtime. As a result, the components P (plants) and E (ears) are sometimes merged into a single unit for practical evaluations.
Potential Seed Yield vs. Actual Yield
Potential seed yield is driven by the number of stems (or flowers on a stem) and is represented as:
Potential seed yield = Number of stems × Seed weight.
In terms of actual yield, we must also consider how many seeds each stem produces. A good yield is typically when a quarter to half of the flowers yield seeds.
Example of Ryegrass Seed Yield
When calculating potential seed yield for perennial ryegrass:
It involves analyzing:
Number of seed heads per square meter.
Number of florets on a seed head.
Individual seed weight (1000 seed weight divided by 1000).
Typical observations show ryegrass can have between 1,500 to 2,500 seed heads per square meter, with 20-30 spikelets per head and an individual seed weight ranging from 1.8 to 2.2 grams.
The simplified calculation, assuming 2,000 heads, 20 spikelets, and an average weight of 2 grams, results in a potential seed yield of:
The highest recorded yield for ryegrass is 4,000 kg per hectare, suggesting farmers usually produce seed yields between 1,600 and 3,000 kg/hectare, with yields above 3,000 kg/hectare being exceptional. Farmers typically achieve 2-3 seeds per floret out of the five to eight flowers expected to yield seeds.
Factors Influencing Seed Yield
Several conditions drive the number of fertile heads at harvest:
Sowing Date: Key for slow-establishing species like tall fescue, but less critical for ryegrass.
Nitrogen Fertilization: Particularly vital is the nitrogen application early in spring; the more nitrogen, the higher the number of fertile tillers produced. Nitrogen rate experiments show strong positive effects on seed yield.
Defoliation Timing: Late defoliation risks removing growing points, which may lower the number of fertile tillers.
Lodging: As seed heads develop, the weight can cause the plant to fall over, leading to the abortion of many developing seeds at harvest. Plant growth regulators may be used to prevent lodging.
Weed Competition: While generally managed well by farmers, weeds can still reduce tiller numbers. In multi-year crops, controlling volunteer plants from previous harvests is vital to prevent competition.
Drought Stress: Affects late-flowering species the most, impacting reproductive units.
Post-Harvest Management: Decisions regarding the stubble, including baling or burning, can influence light capture for regrowth of tillers.
Genetic Factors: New cultivars may not yield as many seed heads or be more challenging to manage.
Traditional Observations
Historically, low seed head numbers contributed to low seed yields, but modern crop management typically resolves these issues, as setups are more efficient.
Flower and Seed Development
Count of spikelets per tiller in grasses and flowers per stem in clover is generally consistent and influenced genetically. However, farmers can influence the efficacy of how many flowers produce marketable seeds, captured by the term "florid site utilization" (the proportion of florets producing seeds).
In ryegrass, this proportion typically ranges from 10% to 40%.
Pollination Implications
Pollination rarely limits seed production in grasses. It's essential to note that although 80% of flowers may develop baby seeds, not all mature into saleable seeds due to various reasons, including:
Seed Abortion: Occurs when developing seeds are not adequately nourished. This could stem from competition among seeds for nutrients or limitations in seed size due to nutrient shortages.
Environmental Stressors: Includes moisture stress, heat, and frost during flowering, which may kill embryos or affect seed viability.
Harvest Index Concept
Harvest Index (HI) indicates the grain or seed weight as a proportion of total dry matter available at harvest. In cereals, an HI of about 50% is expected, while for forages and vegetable seeds, it is between 15-20% due to plants being bred for leaf production rather than seed.
Understanding harvest index is crucial when dealing with cereals, less so for forages where numbers are generally lower.
Factors Leading to Low Florid Site Utilization
Causes include:
Pollination failure, particularly in legumes.
Genetic limitations where cultivars produce fewer seeds.
Nutrient competition leading to seed abortion and underdevelopment.
Poor environmental conditions affecting seed viability.
Carbohydrate Dynamics and Seed Filling
Research indicates a critical relationship between carbohydrate production and transfer to seeds:
The process of photosynthesis produces carbohydrates which are transported to the seed head and converted to starch.
A distinction arises between "source-limited" conditions (where the plant can't produce enough) and "sink-limited" conditions (where seeds fail to absorb sufficient nutrients).
Studies showed ample carbohydrate levels, highlighting that the problem often lies with nutrient uptake at the seed level rather than the production from the plant.
Seed Development Dynamics
Seed development occurs sequentially from the base of the spikelet to the tip, where earlier seeds receive more nutrients, resulting in disparities in size. Environmental factors and plant genetics regulate flowering timing, which in turn influences seed filling efficiency.
Implications of Vegetative Growth
Excess vegetative growth, particularly from nitrogen over-application, can suppress seed development and yield. Balancing between vegetative and reproductive growth phases is essential for optimizing seed production.
Conclusions and Future Directions
Monitoring plant growth at different stages, including flowering to harvest, aids in determining effective management strategies to enhance yields.
The ongoing developments in digital technologies within agricultural fields, as shared in upcoming discussions, may provide further insights and optimizations in managing seed crop yields.
Upcoming Topics
Further discussions regarding companion crops will explore ryegrass, white clover, and carrot cultivation strategies.